Method and device for fabricating aerogels and aerogel monoliths obtained thereby

ABSTRACT

Method and devices for rapidly fabricating monolithic aerogels, including aerogels containing chemical sensing agents, are disclosed. The method involves providing a gel precursor solution or a pre-formed gel in a sealed vessel with the gel or gel precursor at least partially filling the internal volume of the vessel and the sealed vessel being positioned between opposed plates of a hot press; heating and applying a restraining force to the sealed vessel via the hot press plates (where the restraining force is sufficient to minimize substantial venting of the vessel); and then controllably releasing the applied restraining force under conditions effective to form the aerogel. A preferred device for practicing the method is in the form of a hot press having upper and lower press plates, and a mold positioned between the upper and lower plates. Doped aerogel monoliths and their use as chemical sensors are also described.

This application is a continuation of U.S. patent application Ser. No.10/926,901, filed Aug. 26, 2004, now U.S. Pat. No. 7,384,988 whichclaims the priority benefit of U.S. Provisional Patent Application Ser.No. 60/498,329, filed Aug. 26, 2003, each of which is herebyincorporated by reference in its entirety.

STATEMENT OF GOVERNMENT SPONSORSHIP

The present invention was made with funding received from the NationalScience Foundation under grant CTS-0216153. The U.S. government hascertain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to methods and devices forfabricating aerogels, monolithic aerogel products, and their use.

BACKGROUND OF THE INVENTION

Silicate precursors, such as tetramethoxysilane (Si(OCH₃)₄, “TMOS”), arecommonly used to produce porous silica glasses (Brinker et al., Sol-GelScience, Academic Press, New York (1990)). The resultant materials arehighly crosslinked polymer matrices with solvent filled pore spaces.This solvent can be evacuated from the pore-matrix either by evaporationor by supercritical extraction; the two methods yield materials withdifferent physical properties. During solvent evaporation, surfacetension, which exists at any liquid-vapor interface, exerts a forcelarge enough to collapse the pore structure until the gel networkbecomes strong enough to resist this compressive force (Brinker et al.,Sol-Gel Science, Academic Press, New York (1990)). This processgenerates a condensed silicate matrix, referred to as a xerogel, whichis made up of 60-90% air and has pore diameters of 1-20 nm (Brinker etal., Sol-Gel Science, Academic Press, New York (1990)). Evacuating thesolvent above its critical point, where neither liquid nor gas ispresent, can eliminate the surface tension. Consequently, the porestructure does not collapse, but is instead maintained, yielding alow-density solid known as an aerogel (Pierre et al., Chem. Rev.102:4243 (2002)). Aerogels typically consist of 90-99% air (Lev et al.,J. Gun, Anal. Chem. 67:22A (1995)), with pore diameters from 1-50 nm(Brinker et al., Sol-Gel Science, Academic Press, New York (1990)).

The resulting aerogel structure is responsible for giving aerogelmaterials claim to the lowest known density, index of refraction,thermal, electrical, and acoustical conductivities of any solidmaterial. First discovered by Kistler (J. Phys. Chem. 63: 52 (1932)) inthe 1930s, many attempts have been made to take advantage of theirunique properties. Current application areas include Cerenkov radiationdetectors (Ganezer et al., IEEE Trans. Nucl. Sci. 41:336 (1994);Hasegawa et al., Nucl. Inst. Phys. Res. 342:383 (1994)), electronics(Hrubesh et al., J. Non-Cryst. Solids 188:46 (1995)), thermal insulators(Reiss, Phys. Blaetter 48:617 (1992)), insulated windows (Hrubesh etal., J. Non-Cryst. Solids 188:46 (1995); Lampert, Int. J. Energy Res.7:359 (1983)), comet dust collectors (Tsou, J. Non-Cryst. Solids 186:415(1995)), and heat storage devices for automobiles (Fricke et al., J.Sol-Gel Sci. Technol. 13:299 (1998)).

At present, aerogel materials are difficult and expensive tomanufacture. It can take days to weeks to make an unbroken monolith.This manufacturing complexity has limited their development incommercial applications. Aerogels are typically formed in a two-stepprocess. The first step is to form a wet gel by a sol-gel polymerizationreaction. The second step is to extract the solvent and dry the wet gelto form the aerogel.

The primary challenge in the fabrication of an aerogel is to prevent thecollapse of the porous structure during the drying phase. Stresscontributors such as thermal gradients and pressure concentrations aresignificant in aerogel fabrication, but relatively easy to minimize.More difficult to control, however, are the capillary stresses fromsurface tension that, for the nanoscale pore structure, are strongenough to cause structural collapse. As the sol-gel dries, thesecapillary forces can result in significant fracture to the structure.The current methods used to avoid fracture in aerogel fabrication can becategorized into three general techniques, although each drying protocolis designed to minimize or eliminate surface-tension effects. They are(1) ambient pressure techniques; (2) conventional supercriticalextraction (CSCE) techniques; and (3) rapid supercritical extraction(RSCE) techniques.

The ambient-pressure techniques attempt to dry the wet gel at ambientpressure. To do so they require chemical processes to reduce thecapillary forces. One method is to treat the surface of the gel with asurfactant, or surface-tension-reducing chemical (see, e.g., Yusuf etal., J. Non-Cryst. Solids 285:90 (2001); or Lev et al., Anal. Chem.67:22A (1995)). Another technique used by Hereid et al. (J. Sol-Gel Sci.Technol. 3:199 (1994)) ages the gels in alkoxide/alcohol solutions tostiffen the microstructure and avoid collapse due to capillary forces. Atechnique developed by Prakash et al. (J. Non-Cryst. Solids 190:264(1995)) manipulates the surface chemistry of the gel to aid in thesolvent evacuation. This method uses a solvent exchange with hexane,followed by a surface modification with a silylation process to promotea reversible shrinkage. These techniques have been used successfully inthe fabrication of aerogel films, but have had limited success foraerogel monoliths.

The conventional supercritical extraction techniques (CSCE) aremulti-step techniques designed to eliminate surface tension altogetherby bringing the sol-gel to the critical point of the solvent. Above thecritical point there is no surface tension, and the solvent can beevacuated without damage to the gel structure. The technique firstdeveloped by Kistler (J. Phys. Chem. 63: 52 (1932)) entailed two steps:the formation of the wet gel, and the subsequent solvent evacuation in aheated pressure vessel at the supercritical conditions. The mainlimitations of this technique are the difficulties associated withobtaining the high temperatures necessary to reach the critical point ofthe alcohol solvent, as well as the safety concerns with operating thepressure vessel at those conditions. In response to these concerns, alengthy solvent exchange with liquid CO₂ can be performed prior tosupercritical extraction, which can then take place at the criticalpoint of CO₂ (see, e.g., Tewari et al., in Aerogels, J. Fricke (Ed.),Springler-Verlag, New York (1986), p. 31; Van Bommel et al., J.Non-Cryst. Solids 186:78 (1995)). The advantages of the CSCE method area lower critical temperature and a more stable solvent; however anadditional step is added to the process. Because the critical pressurerequirement is not changed significantly, this process still requiresthe use of thick pressure vessels and places practical limitations onthe maximum size of the aerogel. In addition, the solvent-exchangeprocess becomes a size deterrent, as the diffusion kinetics of thesolvent exchange depend upon the size of the gel. Even if a pressurevessel were available to contain a large monolith, the solvent exchangecould take weeks to complete, depending on the size of the monolith.

The third technique, rapid supercritical extraction (RSCE), wasdeveloped by Poco et al. (Mat. Res. Soc. Symp. 431:297 (1996)) anddescribed further in Scherer et al. (J. Non-Cryst. Solids 311:259(2002)). Similar to the CSCE techniques, RSCE is a supercriticaltechnique designed to perform the solvent extraction under supercriticalconditions. In contrast to the CSCE techniques, however, the RSCE is aone-step, reactant-to-aerogel process. The liquid precursor chemicalsand catalyst are inserted into a two-piece mold that is then heatedrapidly to speed up the polymerization. The pressure is initially set byfastening the two mold parts together with properly tensioned bolts, orby applying an external hydrostatic pressure inside of a larger pressurevessel, or by a combination of these two. Once the supercritical pointof the alcohol is reached, the supercritical fluid is allowed to escapethrough gaps formed by the roughness in the surface contact between thetwo portions of the mold, or through a relief valve set just above thesupercritical pressure. A benefit of this method is that the entireprocess is done in one step, and can be accomplished in under an hour,as opposed to multiple steps (and time scales on the order of weeks) forall other available methods.

The advantage of the ambient-pressure methods is that they do notrequire expensive and potentially dangerous pressure equipment. They arecurrently being used successfully in the fabrication of aerogel powdersand thin films. For the fabrication of monolithic pieces, however, thistechnique has yet to prove reliable. Conventional supercriticalextraction has been used extensively in the fabrication of very largeaerogel monoliths, however it can take days to weeks to make them, andthe required multiple steps make the process complicated. In addition tothe reduction in fabrication time, the rapid supercritical extraction asa one-step process has the most potential for reliable and repeatablefabrication, as well as increased production volume.

The present invention relates to a fast supercritical extractiontechnique for fabricating aerogels that overcomes the above-identifieddeficiencies of the prior art.

SUMMARY OF THE INVENTION

A first aspect of the present invention relates to a method for rapidlyproducing aerogels that includes the steps of: providing a gel precursorsolution or a pre-formed gel in a sealed vessel with the gel or gelprecursor at least partially filling the internal volume of the vesseland the sealed vessel being positioned between opposed plates of a hotpress; heating and applying a restraining force to the sealed vessel viathe hot press plates; and controllably releasing the applied restrainingforce under conditions effective to form the aerogel.

A second aspect of the present invention relates to a method for rapidlyproducing aerogels that includes the steps of: heating and applyingexternal restraining force to a sealed vessel that contains therein agel precursor solution or a pre-formed gel that at least partially fillsthe internal volume of the sealed vessel, said heating and applyingexternal restraining force being carried out without substantial ventingof the vessel and thereby confining physical expansion of the gel or gelprecursor; and controllably releasing the external restraining forceapplied to the vessel, thereby allowing for venting and release ofinternal pressure to form the aerogel.

A third aspect of the present invention relates to a device for formingaerogels that includes: a hot press having upper and lower press plateswhich can be manipulated toward and away from one another to apply orrelease force on a mold therebetween; and a mold positioned between theupper and lower plates of the hot press, the mold being adapted toconfine an aerogel or precursors of the aerogel.

A fourth aspect of the present invention relates to an aerogel that isformed according to the first or second aspects of the presentinvention. A preferred form of aerogel contains embedded within themonolith one or more chemical sensing agents, such as a fluorescent dyeor a fluorescent coordination complex.

A fifth aspect of the present invention relates to a chemical sensorthat includes: a monolithic aerogel that contains one or more chemicalsensing agents; a light source that illuminates the monolithic aerogelat an excitation wavelength of the chemical sensing agent; and adetector that detects an emission signal from the chemical sensing agentor transmitted light.

A sixth aspect of the present invention relates to a method of detectinga chemical in gas phase that includes the steps of: exposing a gassample to the chemical sensor according to the fifth aspect of thepresent invention; and detecting a change in agent-emitted light ortransmitted light, the change indicating presence of the chemical in thegas sample.

The process of the present invention offers a number of distinctadvantages over existing techniques. The process is fast, simple, andeasily automated, which may make it more amenable to large-scaleassembly-line fabrication applications. The process is also safe. Thehydraulic press provides a restraining force capable of (1) containingthe internal pressure and (2) controllably releasing excess pressure.The only pressurized volume is the gel itself, not an entire hydrostaticvolume as is the case when using an autoclave or an external hydrostaticpressure, as in the RSCE process. Additional safety concerns associatedwith the release of supercritical alcohol can be alleviated by encasingthe inter-platen working area and flushing the working area with anitrogen purge to eliminate the possibility of auto-ignition of hotsolvent (e.g., methanol) should it escape from the mold and mix withair. By doping the aerogel precursors with fluorescent dyes orcoordination complexes, it is possible to prepare chemical sensors thatare cost- and time-effective, require little synthetic work, and can beeasily adapted for simultaneous use with many of the current aerogelsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a mold and press configuration foraerogel processing. A preferred configuration includes a stainless-steelaerogel mold sandwiched between Kapton and a high-temperature gasketmaterial. The hydraulic hot press provides a restraining force duringprocessing.

FIG. 2 is a graph illustrating the in-mold temperature (thick line) andpressure (thin line) data for aerogel processing shows the temperatureramp, soak and cool down processes that occur during a 5 hour cycle. Forthis test, the temperature ramp rate was set at 1.1° C.min⁻¹ and cooldown rate at ˜5° C.min⁻¹.

FIG. 3 is a graph illustrating the pressure versus temperature data forthe 5 hour process depicted in FIG. 2, and shows that the solution staysabove the methanol liquidus line (indicated by dashed line) untilcritical temperature and pressure are reached.

FIGS. 4A-B illustrate two aerogel monoliths produced by the method ofthe present invention. The monolith shown in FIG. 4A is about 25.4 mmdiameter×12.5 mm high. The monolith shown in FIG. 4B is about 24 mmdiameter×17 mm high.

FIG. 5 is a graph illustrating the Barrett-Joyner-Halenda (“BJH”)Desorption dV/dlogD pore-volume data for two sample aerogels fabricatedin accordance with the present invention. The solid lines are used tovisually connect the data and do not represent an analytical relationbetween pore diameter and pore volume.

FIG. 6 is a graph illustrating the transmission results as a function ofwavelength of light for two aerogel samples, which both showtransmission of 60-90% in the near-infrared region.

FIGS. 7A-C illustrate different mold constructions that have been usedto prepare aerogels using the process of the present invention. FIG. 7Aillustrates a four hole design, each hole preparing a single 1.5 inchdiameter×¾ inch high aerogel. FIG. 7B illustrates a single hole design,the hole preparing a single ¾ inch diameter×1 inch high aerogel. FIG. 7Cillustrates a twenty-five hole design, each hole preparing a ⅜ inchdiameter×¾ inch high aerogel.

FIGS. 8A-C illustrate the structures of oxygen-sensitive probes used inExample 6. FIG. 8A shows the structure oftris(2,2′-bipyridyl)ruthenium(II); FIG. 8B shows the structure ofruthenium(II) 4,7-diphenyl-1,10-phenanthroline; and FIG. 8C shows thestructure of platinum octaethylporphine.

FIGS. 9A-C illustrate a gas-mixing system. In FIG. 9A, gaseous nitrogenand ultrapure air (1) are delivered to TYGON® tubing via two-stageregulators (2), the gases are mixed in a gas proportioner (3), and thenpass through a line regulator (4) and an injection port (5) to afluorimeter (6). In FIG. 9B, the gas mixture then flows through inletport (5) and the tubing is fed into a cuvette through a modified cuvettecap (7). The sample is held in the cuvette (8). The gas mixture isvented into the fluorimeter chamber through a piece of TYGON® tubing(9). FIG. 9C shows two holes bored into the top of a cuvette cap (7).TYGON® tubing was threaded snugly through those holes.

FIG. 10 is a graph illustrating the reversible response of Ru(bpy)₃ ²⁺-and Ru(dpp)₃ ²⁺-doped aerogels to changes in ambient O₂ (g)concentration. The response of the Ru(bpy)₃ ²⁺-doped aerogel wasnormalized to that of the Ru(dpp)₃ ²⁺-doped aerogel. Each sample wasinitially in air under ambient conditions in an uncapped cuvette. Atapproximately t=60 s, N₂ (g) was allowed to impinge directly onto thesample. The N₂ (g) was then shut off at 120 s (a) and 180 s (b only). Anadditional cycle is shown in run (b). λ_(ex)=446 nm, λ_(em)=615 nm, 10data pts/s.

FIG. 11 is a graph that illustrates a time-based scan showing responseto oxygen and reversibility of Ru(dpp)₃ ²⁺-doped silica aerogel sensor.Each step represents a certain percentage of oxygen. The scan was takenusing λ_(ex)=446 and λ_(em)=615 nm.

FIG. 12 is a graph that illustrates a time-based scan showing responseto oxygen and reversibility of PtOEP-doped silica aerogel sensor. Eachstep represents a certain percentage of oxygen. The scan was taken usingλ_(ex)=533 and λ_(em)=646 nm.

FIG. 13 is a graph illustrating Stern-Volmer plots for Ru(dpp)₃ ²⁺-dopedsilica aerogel (♦), xerogel (▴), and post-doped xerogel (▪).

FIG. 14 is a graph illustrating a Stern-Volmer plot for PtOEP-dopedaerogel.

FIG. 15 is a top plan view of a gas sensor device that utilizes a dopedaerogel containing a chemical sensor agent.

FIG. 16 is a side elevational view of the gas sensor device shown inFIG. 15, with one of the detectors omitted to expose the housingbeneath.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to methods and devices for theone-step fabrication of monolithic aerogel products.

The device used in accordance with the processes of the presentinvention generally includes a hot press that has upper and lower pressplates, and a mold positioned between the upper and lower plates of thehot press.

The hot presses that can be employed in the present invention caninclude either hydraulic or electronic regulation of the upper and lowerpress plates. One requirement for the hot press is that thehydraulically or electronically regulated press plates can bemanipulated toward and away from one another to apply or release on amold therebetween a restraining force that is sufficient to preventand/or minimize any venting during the ramp-up and dwell phases ofaerogel production. The heating mechanism employed on the hot press ispreferably an electrical heating system that can achieve heating up toat least about 300° C., at rates of up to about 2° C./min. The hot pressalso needs to provide a restraining force sufficient to counter theinternal pressure force created by the precursors in the mold, typicallyup to about 220 kN (50,000 lb). The hot press platen surfaces should besubstantially parallel and flat to within at least about 0.13 mm (0.005inches).

By way of example and without limitation thereto, a preferred type ofhot press is 50-ton hydraulic hot press manufactured by AccudyneEngineering and Equipment Company (Bell Gardens, Calif.). Other suitablehot presses include those manufactured by Tetrahedron, Inc.

The mold includes a mold body and upper and lower seals positionedagainst upper and lower surfaces of the mold body. During use, the upperand lower seals contact, respectively, the upper and lower press platesof the hot press.

The mold body is preferably formed of a substantially smooth surfacedmaterial that can sustain the thermal cycling without deformation, andhas thermal properties that allow for uniform heating of the aerogelprecursor or aerogel contained therein. Exemplary materials used forforming the mold include, without limitation, stainless steel, steel andaluminum.

The mold can be shaped and configured to any desired size or shape ofthe resulting aerogel monolith that is to be fabricated. The mold cancontain any number of receptacles that are designed to retain individualaerogel or aerogel precursors (i.e., two or more) provided the wallthickness around each receptacle is sufficient to prevent deformationduring processing. The mold surface, which contacts the seal (discussedbelow), is preferably a substantially planar surface, flat within about0.013 mm (0.005 inches). The substantially planar surface of the moldensures that a tight seal can be formed between the mold surface and thepress plates, thereby minimizing or substantially precluding venting ofthe mold during ramp-up and dwell phases of the process. The moldsurface can be sprayed with mold release, such as high temperatureTeflon spray, to facilitate removal of the aerogels from the mold;however, this is not required for the process.

The seal against the upper and lower press plates is formed of acomposite gasket that includes a high temperature gasket materialsandwiched between high temperature sealant material. The sealantmaterial is used to prevent the high temperature gasket material fromadhering to the mold or platen surface. It is preferably about 0.025 toabout 0.50 mm (1 to 2 mil) thick Kapton™ or Teflon™. Of these, Kapton™is preferred. The high temperature gasket material is preferably about0.8 to about 1.6 mm ( 1/32 to 1/16 inch) thick graphite, silicone rubberor Teflon™. Of these, graphite is preferred.

Aerogel precursor materials are widely known in the art, and anysuitable aerogel precursor materials can be used to form aerogelsaccording to the present invention. Known aerogel materials includesilica, alumina, titania, hafnium carbide, various polymers, andchalcogenide semiconductors (e.g., CdSe, ZnS, PbS). By way of example,silica aerogels can be formed with tetralkoxysilanes such as TMOS incombination with alcohols such as methanol or ethanol, water, and a basesuch as ammonium hydroxide. The ratio of these materials can be adjustedas is known in the art to achieve aerogel properties that are optimizedfor particular end uses thereof.

In addition to the aerogel precursor materials, the aerogel precursorcan be doped to include one or more chemical sensing agents. Any knowndopants of the type used for thin sol-gel film sensors can be utilizedas long as the dopants are stable at temperatures used in the processdescribed hereinafter. Suitable chemical sensing agents includefluorescent dyes and fluorescent coordination complexes, and can beemployed to interact with another chemical species of interest, forsensing applications, or used for fundamental studies of aerogelmicrostructure. Exemplary organic dyes include, without limitation,fluorescein, rhodamine B, rhodamine 6G, and8-hydroxypyrene-1,3,6-trisulfonic acid. Exemplary fluorescentcoordination complexes include, without limitation,tris(2,2′-bipyridyl)ruthenium(II) or Ru(bpy)₃ ²⁺, ruthenium (II)4,7-diphenyl-1,10-phenanthroline or Ru(dpp)₃ ²⁺, and platinumoctaethylporphine (PtOEP).

In practicing the method of fabricating aerogel monoliths in accordancewith the present invention, a gel precursor solution or a pre-formed gelis first provided in a sealed vessel (i.e., the mold) with the gel orgel precursor at least partially filling the internal volume of thevessel and the sealed vessel being positioned between opposed plates ofa hot press. The at least partially filled vessel can either be (1)filled, sealed, and then transferred to a position between the pressplates or (2) positioned between the plates (i.e., supported by thelower plate), filled, and then sealed. Regardless of the approach usedfor filling and sealing the vessel, once the vessel has been positioned,aerogel formation can begin.

Aerogel formation is performed by heating and applying a restrainingforce to the sealed vessel via the hot press plates. The heating andapplying of restraining force can be carried out simultaneously via thehot press plates; or the application of restraining force can be carriedout prior to heating.

The restraining force is an external force (applied by the hot press viathe plates) sufficient to counter the forces caused by the pressureincrease in the sealed vessel that results from heating of the aerogelprecursor materials. The restraining force to be applied will dependupon both the size of the mold and the amount of pressure which isexpected to develop inside the vessel during the heating process. Theestimated restraining force (F_(rest)) can be defined as:F _(rest) =P _(mold) *A _(mold)  Eq. (1)where P_(mold) is the pressure in the mold, typically between 9 MPa and17 MPa (1300 to 2500 psi) and A_(mold) is the surface area of the mold.In general, the restraining force is at least about 70 kN.

By way of example, a restraining force of at least about 145 kN (32,500lb), most preferably between 145 kN to about 280 kN is appropriate for a5 inch by 5 inch mold containing four 1.5 inch-diameter receptacles.

The heating of the mold and aerogel precursor materials is carried outuntil a temperature above the supercritical temperature of the aerogelsolvent is achieved. This is the ramp-up phase of heating. The ramp-upphase is carried out at a rate that is suitable to allow for aerogeldevelopment. Preferably the heating rate is up to about 2° C. perminute, more preferably between about 1° C. to about 2° C. per minute.The maximum temperature to be achieved is typically, though notexclusively, between about 240° C. to about 300° C.

Once the maximum temperature has been achieved, the temperature canoptionally be maintained for a dwell or soak phase. During the optionaldwell or soak phase, the restraining force applied to the sealed vesselis also preferably maintained (i.e., it is preferred that no ventingoccurs during this phase). The dwell or soak phase can be of anyduration, more preferably between about 1 minute and about 60 minutes,most preferably between about 10 and about 30 minutes.

It is preferable that no substantial venting occurs during either theramp-up phase or the optional dwell phase; however, some venting mayoccur to the extent that the internal pressure within the mold overcomesthe restraining force applied externally of the mold via the hot pressplates. To the extent that venting does occur (i.e., the restrainingforce is low enough to allow venting to occur), such venting should notallow internal pressure within the mold to drop below the criticalpoint. Hence, the restraining force to be applied should be selected toalways maintain the solvent in the supercritical state during ramp-upand any optional dwell phase.

After any optional dwell phase, the applied restraining force iscontrollably released under conditions effective to remove any remainingsolvent from the sol-gel, thereby forming the aerogel. By controllablyreleasing, it is intended that a portion of the restraining forceoriginally applied to the sealed vessel via the hot press plates isremoved. The remaining restraining force is insufficient to preventventing of the sealed vessel; hence, at this time supercritical fluidescapes from the vessel. By way of example, the remaining restrainingforce is preferably at least about 4 kN, more preferably between about 4and about 30 kN

Upon completion of any dwell phase, the temperature is also decreased toambient temperature. The rate of temperature decrease can be up to about5° C. per minute, more preferably between about 1° C. to about 4° C. perminute, most preferably between about 2° C. to about 3° C. per minute.Upon reaching ambient temperature, any remaining restraining force isremoved. Thus, the plates of the hot press are retracted, allowing forcollection of the resulting aerogels.

It is important to note that the time required to prepare the aerogelsin accordance with the present invention is preferably not more thanabout 12 hours, more preferably not more than about 9 hours, and mostpreferably not more than about 6 hours. Process times of as little asapproximately 5 hours have been achieved.

As noted above, the aerogel can be doped with one or more chemicalsensor agents. Aerogels doped with such probes are particularly usefulin preparing chemical sensors that are capable of detecting a chemicalspecies in, e.g., a gas sample. Exemplary chemical species that can bedetected include, without limitation, oxygen, carbon dioxide, carbonmonoxide, and hydrocarbons.

The chemical sensor will include a doped aerogel of the presentinvention, a light source that illuminates the monolithic aerogel at anexcitation wavelength of the chemical sensing agent, and a detector thatdetects an emission signal from the chemical sensing agent ortransmitted light. Depending upon the position of the detector relativeto the light source, the detector can measure either emissions from thechemical sensing agent (i.e., scattered light) or transmitted light thatpasses through the doped aerogel. In the latter embodiment, the detectoris directly opposite the light source. In the former embodiment, thedetector is laterally displaced from the light source such that it isnot illuminated directly. Optionally, both types of detectors can beused simultaneously; that is, at least two detectors are present.

Suitable light sources include, without limitation, light emittingdiodes, laser diodes, and any white light source such as flashlightbulbs (an incandescent bulb).

Suitable detectors include, without limitation, photodiodes,photomultiplier tubes, and charge-coupled detectors.

In addition to the light source and detectors, the sensor device canalso include a housing that contains the monolithic aerogel and isprovided with a passage for delivering a gas sample across the aerogel.The gas sample can be delivered passively or actively. The sensorhousing should be optically transparent, at least in the range of lightthat used for illumination and for the range of light that is emitted bythe chemical sensing agent.

One or more filters can also be utilized, either between the monolithicaerogel (i.e., the housing containing the same) and the light source,between the monolithic aerogel (i.e., the housing containing the same)and the detector, or both. Suitable filters include bandpass filters andlongpass filters. It is preferable that the filter between the aerogeland the light source is a bandpass filter, whereas the filter betweenthe aerogel and the detector(s) can be either a longpass filter or abandpass filter.

The chemical sensor device can be coupled to an electrical controlsystem that is designed to monitor the output of the detectors and, ifnecessary, sound an alarm when the quantity of the sensed chemical agentbecomes too high or merely when its presence is detected. In the formersense, the sensor is a quantitative sensor. In the latter sense, thesensor is used merely as a non-quantitative switch.

An exemplary embodiment of the chemical sensor device is illustrated inFIGS. 15 and 16. In this embodiment, a doped aerogel forms the sensorelement 50, which is present in an optically transparent housing 52. Asshown in FIG. 16, the housing includes inlet 64 and outlet 66 passagesfor exposure of the aerogel to a vapor sample. The light source 54(e.g., light emitting diode or laser diode) is positioned on one side ofthe housing, and three detectors 58, 58′ (e.g., photodiodes) arepositioned on three sides of the housing. The detector directly acrossfrom the light source is positioned to detect transmitted light. Thedetectors displaced 90 degrees from the light source are positioned todetect scattered light, such as that emitted by the chemical sensingagent present within the aerogel monolith. A bandpass filter 56 isprovided between the housing and light source, as well as between thehousing and detector 58. A longpass filter or bandpass filter 60 isprovided between the housing and the detectors 58′ to block light fromexcitation source, but transmit fluorescence intensity from the chemicalsensing agent.

In use, the chemical sensor will be exposed to a gas sample that maycontain the chemical species to be detected, and a change inagent-emitted light or transmitted light is detected after suchexposure. Any change can indicate presence of the chemical species inthe gas sample. As noted above, the sensor can in some circumstances beused to detect quantitatively the amount of the chemical species that ispresent in the gas sample.

EXAMPLES

The following examples are intended to illustrate, but by no means areintended to limit, the scope of the present invention as set forth inthe appended claims.

Example 1 System for Aerogel Fabrication

Referring to FIG. 1, equipment used to fabricate the aerogel monolithsby the new fast supercritical extraction technique of the presentinvention includes a 50-ton hydraulic hot press (having upper and lowerpress plates 40, 40′), a stainless-steel mold 42, and a high-temperatureseal 44 formed of a graphite gasket 46 sandwiched between sheets ofKapton™ film 46. By way of example, the mold 42 is fabricated from a10×10×2 cm³ thick piece of stainless steel (see FIG. 1). Sixteen 2.2 cmdiameter, 1.7 cm-deep holes (with a 7° taper) are spaced uniformlyacross the plate. The surface was ground flat to ensure a good sealbetween the gasket and the mold.

Example 2 Fabrication of Aerogels

The system described in Example 1 above was used to prepare aerogels.

The one-step fabrication process entails two main phases, both of whichtake place inside of the mold during a continuous process. The firstphase is to polymerize silica to form a sol-gel, and the second phase isto safely evacuate the water and alcohol solution. Initially, thechemical precursors for the aerogel were mixed and poured into the mold.For a tetramethoxysilane-derived (TMOS) aerogel, these precursors areTMOS, deionized water, methanol, and ammonium hydroxide with molarratios TMOS:MeOH:H₂O:NH₄OH of 1.0:12.0:4.0:3.7×10⁻³.

The mold is placed in the hydraulic hot press and the processing beginsby applying and holding a 75 kN restraining force. Typically, atemperature ramp of 1.25° C.min⁻¹ is applied to heat the mold fromambient temperature to a supercritical temperature (˜265-300° C.). Theinternal pressure of the mold increases as a result of the temperaturerise as dictated by the thermodynamics of the system. Following a 15-30minute soak at supercritical conditions (for methanol, T_(crit)=240° C.and P_(crit)=8.1 MPa), the load is dropped to 4 kN, allowingsupercritical fluid to escape, and the temperature is decreased at arate of up to about 2.5° C.min⁻¹. Upon reaching ambient temperature, theplatens of the hydraulic press retract, and the resulting aerogels arecollected. The overall time to complete the process as described is 5hours.

Process temperature and pressure data were acquired by instrumenting themold with a melt pressure transducer. FIG. 2 plots the temperature andpressure data as a function of time and FIG. 3 plots pressure versustemperature superimposed on a methanol liquidus line (indicated by thedashed line). The temperature ramp phase lasted 3 hours. As the moldtemperature increased the pressure rose to approximately 21 MPa. At thispoint some of the gases were released and the pressure dropped to about13 MPa, which is still above the critical point. (This step demonstratesthat the press is capable of acting as a pressure-relief valve when theinternal pressure gets too high.) After about a 15 minute soak, thepress restraining force was dropped to 4 kN, allowing for evacuation ofthe supercritical gases and rapid dropping of the internal mold pressureto ambient pressure while maintaining a high temperature. In the finalstage, the press was cooled down to ambient temperature in 1.5 h. FIG. 3shows that the process successfully avoids crossing the methanolliquidus line.

The process is a one-step precursor to aerogel method. Duringprocessing, the specimen experiences the stages of gelation,aging/strengthening, solvent extraction, and cooling to ambienttemperature. Gelation refers to the polymerization reaction that resultsin a solid network suspended in the liquid solvent. The specimen is saidto have gelled when the polymer network has spanned all sides of thecontainer, although the polymerization reactions continue as theproducts approach equilibrium compositions. The post-gelationpolymerization is referred to as the aging and strengthening stage. In aseparate gelation study, it has been demonstrated that gelation occursin the first 120 minutes of the temperature ramp phase. During theremaining 75 min of the ramp and during the upper-temperature soak, thegel has a chance to strengthen and age. Although thermal stresses are aprimary concern in setting the temperature ramp rate, the process mustallow sufficient time for the gelation and aging/strengthening stages.

The processing cycle eliminates the capillary forces that cause fractureby avoiding liquid-gaseous interfaces. This control is achieved bystaying on the liquid side of the methanol liquidus curve until thecritical temperature and pressure are reached (as shown in FIG. 3). As asupercritical fluid, the processing curve can go around the liquiduscurve (without crossing it) to ensure that the removal of the liquidphase does not take place in the presence of a gas phase.

During the high-temperature soak step of the process, the system isbrought to a supercritical temperature and held there to ensure that theentire specimen exceeds the critical temperature. At that point therestraining force on the hydraulic press is relaxed nearlyinstantaneously to a nominal load (4 kN). This nominal load ismaintained to prevent air from entering the mold and oxidizing anyremaining precursor chemicals while at high temperature. As therestraining force is released, the supercritical fluid escapes throughnewly formed gaps between the gasket and the mold. It is important tothe design of the mold gasket assembly that the fluid remains completelytrapped under the restraining force, yet escapes relatively easilyduring the relaxation. For example, the process described above utilizesKapton film to prevent the gasket material from adhering to the platenand mold. After the extraction, the aerogel is formed, but remainsinside the mold at an elevated temperature. At this point the solventhas been evacuated and the cooling path is no longer constrained by theliquidus curve. Thermal stresses are less problematic during the coolingstage because the network has already formed and the solvent has beenevacuated. At present, the cooling rate is limited by the capabilitiesof the hot-press.

The new fabrication technique has proven to be a successful method forfabricating silica aerogel monoliths, forming aerogels in about 5 hourswith thermo-physical properties that are comparable to those of aerogelsmade using prior techniques.

Example 3 Characterization of Aerogels

Using the procedure described in Example 2, several monolithiccylindrical aerogel samples were prepared. Sample aerogel monoliths areshown in FIGS. 4A-B.

Bulk density measurements were made using a caliper and mass balance.Values as low as 0.066 g.cm⁻³ were measured.

The aerogel pore-size data were characterized using a Micromeritics ASAP2010 Nitrogen Adsorption system (Micromeritics Instrument Corporation).Samples were outgassed for 5 hours at 200° C. prior to analysis. A5-point Brunauer-Emmett-Teller (BET) analysis was used to determinesurface area. Mesopore distributions were determined from the desorptionbranch of the nitrogen isotherm using the Barrett-Joyner-Halenda (BJH)method. The BET surface areas were found to be 320 m².g⁻¹, and the BJHdesorption average pore size was found to be about 15 nm. Incrementalpore volume was found to peak at 68 nm. FIG. 5 presents the desorptionpore-size distribution. The 320 m².g⁻¹ surface area is relatively lowfor an aerogel. Poco et al. (Mat. Res. Soc. Symp. 431:297 (1996), whichis hereby incorporated by reference in its entirety) also report low BETsurface areas (compared to CSCE aerogels) for their RSCE aerogels. Theyattribute this to the accelerated gelation phase, which occurs at hightemperatures in the RSCE process and may cause larger necks betweenparticles.

A Hot Disk Thermal Constants Analyzer (HD-01) was used to performaerogel thermal conductivity measurements. This system sensor issandwiched between two pieces of aerogel material and heated by anelectric current for a short period of time. The temperature response ofthe sensor is recorded and the transient record is analyzed to determinethermal properties. The tested aerogel samples had conductivity valuesbetween 30 and 40 mW.mK⁻¹.

A Perkin-Elmer Lambda 900 Series spectrophotometer was used to performthe transmission measurements in the 300 to 2500 nm range. Thetransmission results are presented in FIG. 6 for two aerogel samples.Although both samples were prepared using the same process, one of thesamples was cloudy and had low transmissivity in the visible range. Bothsamples transmitted 60-90% of the light at near-infrared wavelengths(1200 to 2200 nm). Peaks observed in the near-infrared region arecharacteristic of silica sol-gels and agree well with the literature(Venkateswara et al., Mater. Sci. Technol. 14:1194 (1998), which ishereby incorporated by reference in its entirety). Transmission in thevisible region is limited (particularly for the cloudy sample) due toRayleigh scattering, which is particularly significant at lowerwavelengths. As noted in Example 7 infra, the aerogels transmitsufficiently in the visible region to be used as platforms for opticalsensors based on entrapped fluorescent probes.

Example 4 Fabrication of Aerogels

In this example, four 1.5 inch diameter×¾ inch high silica aerogels wereprepared using the mold illustrated in FIG. 7A. The mold was sandwichedbetween two pieces of sealing material ( 1/16 inch thick graphitesandwiched between 2 mil thick pieces of Kapton film) and placed in thehot press, which was then closed and held with a restraining force of 35kN (8,000 lb) for two minutes to seal the bottom of the mold.

A 110 ml batch of precursor solution was prepared using 23.43 ml of TMOS(>98%), 75.58 ml of pure methanol, 10.63 ml of de-ionized water and 0.37ml of 1.5M NH₄OH. The ingredients were mixed and briefly stirred.

The press was then opened, the upper sealing material removed, and theprecursor solution was distributed among the four holes. The uppersealing material was placed back on the mold and the press was closed.The restraining force was set to 180 kN (40,400 lb), and the temperaturewas increased to 288° C. (550° F.) at a rate of 1.2° C./min (2.2°F./min). Upon reaching 288° C. (550° F.), the mold remained at thistemperature for 2 hours.

After two hours, the restraining force was dropped to 26 kN (5775 lb) ata rate of 5.1 kN/min (1150 lb/min). The mold remained at 288° C. (550°F.) and 26 kN (5775 lb) for 2 hours, after which the temperature wasdecreased from 288° C. to 38° C. at a rate of 1.2° C./min. Afterreaching 38° C., the press was opened and the four aerogels wereremoved.

Example 5 Fabrication of Aerogels

In this example, a ¾ inch diameter×1 inch high silica aerogel wasprepared using the mold illustrated in FIG. 7B. The mold was sandwichedbetween two pieces of sealing material ( 1/16 inch thick graphitesandwiched between 2 mil thick pieces of Kapton film) and placed in thehot press, which was then closed and held with a restraining force of 35kN (8,000 lb) for two minutes to seal the bottom of the mold.

A 10 ml batch of precursor solution was prepared using 2.125 ml of TMOS(>98%), 6.875 ml of pure methanol, 0.90 ml of de-ionized water, and0.034 ml of 1.5M NH₄OH. The ingredients were mixed and briefly stirred.

The press was then opened, the upper sealing material removed, and theprecursor solution poured into the hole. The upper sealing material wasplaced back on the mold and the press was closed. The restraining forcewas set to 74 kN (17,000 lb), and the temperature was increased to 288°C. (550° F.) at a rate of 1.2° C./min (2.2° F./min). Upon reaching 288°C. (550° F.), the mold remained at this temperature for 2 hours.

After two hours the restraining force was dropped to 26 kN (5775 lb) ata rate of 1.6 kN/min (364 lb/min). The mold remained at 288° C. and 26kN for 2 hours, after which the temperature was decreased from 288° C.to 38° C. at a rate of 1.2° C./min. After reaching 38° C., the press wasopened and the single aerogel was removed.

Example 6 Preparation of Probe-Doped Aerogels

Tetramethoxysilane (TMOS) was purchased from Sigma-Aldrich at 98%purity. Solutions of the [Ru(bpy)₃]Cl₂.6H₂O dye (Strem Chemicals, 98%purity) were prepared in deionized water. Solutions of the[Ru(dpp)₃]Cl₂.6H₂O dye (GFS Chemicals, unlisted purity) were preparedusing absolute ethanol (CH₃CH₂OH, EtOH), unless otherwise noted. ThePtOEP was purchased from Frontier Scientific. Spectrometric grademethanol was purchased from Aldrich.

The [Ru(dp_(P))₃]Cl₂.6H₂O solid was purified by washing with chilled,distilled deionized water using fine-grade filter paper (FisherScientific), as suggested by Cho and Bright (Anal. Chem. 73: 3289-3293(2001), which is hereby incorporated by reference in its entirety).After drying under ambient conditions, 0.0117 g of [Ru(dpp)₃]Cl₂.6H₂Owas dissolved in 10.0 ml of absolute EtOH to make a 1.0×10⁻³ M Ru(dpp)₃²⁺ stock solution. Additional EtOH solutions of varying Ru(dpp)₃ ²′concentration, ranging from 2.5×10⁻⁷ to 1.0×10⁻⁴ M, were prepared bydilution of the stock solution.

A saturated solution of PtOEP in methanol was prepared by placing 5 mgof PtOEP powder into a 100 ml volumetric flask, which was then filled tothe mark with methanol. (Not all of the powder dissolved, as PtOEP isnot very soluble in methanol.) In preparing sol-gels, 10.00 ml of thePtOEP stock solution was used.

To prepare the sol-gels, TMOS (4.24 ml), methanol (13.70 ml), deionizedwater (2.10 mL), and 1.5 M aq. ammonia (0.067 ml) were combined and thesolution stirred until it was monophasic (˜10 min). Probes wereintroduced as a volume of aqueous solution, in place of a portion of thewater, or as an alcohol solution, in place of a portion of the methanol.The sol-gel-precursor solution was then either (1) poured intodisposable polystyrene cuvettes for xerogel preparation under ambientconditions, or (2) poured into a metal mold (see FIG. 7C) and placedinto a hot press for aerogel preparation using a procedure of the typedescribed in Example 5 supra.

Example 7 Spectroscopic Characterization of Probe-Doped Aerogels

A PTI Steady State Fluorometer System, containing a PTI A-1010 Arc Lamp,a PTI LPS-220B Lamp Power Supply, and a PTI Model 810/814Photomultiplier Detection System, was used to take fluorescenceexcitation and corrected emission spectra, and to collect time-basedemission scans used in gas sensor experiments. A Hewlett Packard modelHP8453 Diode Array spectrophotometer was used to take absorbancemeasurements of the stock solutions and doped sol-gels.

The PTI Steady State Fluorometer System described above was used tocalibrate the response of the doped sol-gels to ambient oxygenconcentration. All measurements were obtained at a fixed excitation andemission wavelength appropriate to the probe.

For the initial sensor response experiments, the sample's fluorescenceintensity was monitored continuously while the sample (in a cuvette) wasalternately exposed to ambient conditions (room air) and flushed withnitrogen from an ultrapure nitrogen gas tank. This method was simple,but it was not suitable for quantitative studies; moreover, the gaspressure on the sample was not carefully regulated, and the resultingchanges in pressure on the samples as the nitrogen tank was turned onand off caused some of the sol-gels to move in the excitation beam. Thisvariable motion was a particular problem for the xerogels, which hadshrunk considerably since gelation.

Subsequently, a gas-mixing system depicted in FIG. 9 was constructed.The ambient oxygen content was controlled by mixing ultrapure air and N₂(g) using a 150 mm Airgas Gas Proportioner. The second-stage regulatorswere set to 20 psi for both N₂ (g) and air. The line regulator wasopened maximally and gave a reading of 17.5 psi. The flow rate into thecuvette was calculated to be 1800±30 mL/min, using the manufacturer'scalibration data for the gas proportioner. A cuvette cap was modifiedfor transport of the gas mixture to and from the sample, as depicted inFIG. 9C.

Immediate success was achieved in entrapping these probes in bothaerogels and xerogels prepared using TMOS. Unbroken cylindrical aerogelmonoliths, ˜1 cm in diameter and 1.5 cm in height, containing each ofthe three probes were obtained via the procedure of the presentinvention. The aerogels appear cloudier than xerogels, indicating asignificant amount of light is being scattered; however, the aerogelmonoliths are sufficiently transparent that no difficulty wasencountered in taking fluorescence measurements.

When Ru(bpy)₃ ²⁺ solutions of 1.0×10⁻⁵, 1.0×10⁻⁴, and 1.0×10⁻³ M wereused in the sol-gel recipe, optically transparent aerogels wereproduced. Higher concentrations yielded opaque aerogels; lowerconcentrations did not give sufficient fluorescence intensity forspectral measurements. Aerogels and xerogels prepared with Ru(dpp)₃ ²⁺stock solutions of 1.0×10⁻⁵ and 1.0×10⁻⁴ M yielded optically usablemonoliths. Undissolved solid PtOEP precipitated out of the PtOEP-dopedsol-gels during the gelation process. The PtOEP precipitate is visibleat the base of the monoliths, but this area is not probed during thespectroscopic measurements.

High-quality absorption spectra of the doped aerogels could not beobtained, because they scattered too much light. Excitation spectra forthe Ru(bpy)₃ ²⁺-doped xerogels and aerogels peaked at 470 nm. Theemission maxima for the aerogels was red-shifted from that of the sameprobe in xerogels: 598-604 nm for various aerogels versus 590 nm for thexerogels.

The fluorescence spectral properties of Ru(dpp)₃ ²⁺ in xerogels andaerogels somewhat depended on fluorophore concentration. The emissionspectrum of the aerogel prepared from 1.0×10⁻⁵ M Ru(dpp)₃ ²⁺ solutionhad a peak at 602 nm, blue-shifted relative to the 615 nm peak observedfor the complex in ethanol. This wavelength is fully consistent with thecorresponding Ru(dpp)₃ ²⁺-doped xerogel. Hence, it is possible that theenvironment(s) experienced by the probe in the xerogel and the aerogelare very similar for these samples.

The UV-visible absorption spectra of the PtOEP stock solution (inmethanol) and wet gels were consistent with the literature (Lee et al.,Analyst 122:81 (1997), which is hereby incorporated by reference in itsentirety). Emission spectra of PtOEP in solution, in aerogels, and inxerogels showed maxima at 646 nm with no significant differences inmaximum emission wavelength. The PtOEP-doped xerogels fluorescedappreciably only in the absence of O₂.

When a Ru(bpy)₃ ²⁺-doped aerogel was exposed to 100% nitrogen, thefluorescence intensity of the entrapped Ru(bpy)₃ ²⁺ underwent a rapidincrease of ˜10%, with the signal stabilizing within 10 s (see FIG. 10).The signal was reversible. The Ru(dpp)₃ ²⁺-doped aerogels gaveconsiderably greater relative response to changes in the percentageoxygen than those doped with Ru(bpy)₃ ²⁺, as can be seen in FIG. 10.Consequently, these samples were explored in greater detail. FIG. 11displays a time-based emission scan of a Ru(dpp)₃ ²⁺-doped aerogel. Thesample showed a quick and reversible response time (<10 s) to theanalyte (oxygen).

Each ‘step’ in this scan represents the fluorescence emission intensityof the doped aerogel at a certain oxygen percentage. For example, thestep between 0 and ca. 175 s represents the emission of the dye-dopedaerogel when exposed to 21.5% oxygen. The O₂ (g) concentration was firstdecreased from 21.5% to 0% O₂ (g) while monitoring the signal intensity;and then increased stepwise to 21.5% while again monitoring. Theresponse was reversible and exhibited strong reproducibility: when theaverage emissions for each percentage step-up and step-down in O₂ areplotted together, the data points are superimposable within one standarddeviation. The fluorescence intensity of the doped aerogel in 100%nitrogen is a factor of 5.4 times greater than when the same sensor isexposed to 21.5% oxygen. However, the Ru(dpp)₃ ²⁺-doped aerogel does notrespond to changes in ambient O₂ (g) concentration in a linear fashion.

The Ru(dpp)₃ ²⁺-doped aerogel was highly sensitive to low O₂ (g)concentrations, with the signal intensity dropping to less thanone-third of its maximum when going from 0 to 3% O₂ (g). The signalresponse of a 1.0×10⁻⁴ M Ru(dpp)₃ ²⁺-doped xerogel was also tested forchanges in ambient oxygen concentration. After applying a positivepressure of N₂ (g), the xerogel signal responded fully and stably after50 s, with a 7.25-fold signal increase. This response was also shown tobe reversible. It is important to note that the 50 s response time wasobtained with a much lower N₂ (g) flow rate than was used for theRu(dpp)₃ ²⁺-doped aerogel, because the xerogel sample physically movedout of the excitation beam when higher flow rates (˜1800 ml.min⁻¹) wereemployed. Response times for the xerogel that was post-doped withRu(dpp)₃ ²⁺ were similar; however, a greater relative change influorescence intensity (factor of 8.6) was observed. For purposes ofcomparison, it should be noted that the 5.0×10⁻⁶ M Ru(dpp)₃ ²⁺ solutionexhibited a 6.3-fold increase in fluorescence intensity after it wasvigorously bubbled with N₂ (g); however, 69 min of deaeration wasrequired to produce a steady response.

The PtOEP-doped aerogel responded to oxygen concentration with aresponse time to increases in oxygen concentration of ˜20 s. Whendecreasing the oxygen concentration, stabilization of the signal wasachieved in about 10 s. FIG. 12 is a time-based scan of the PtOEP-dopedaerogel; it demonstrates the response of the doped aerogel to itsgaseous environment.

Time-based scans of PtOEP-doped xerogels using the same gas mixesdemonstrated that the xerogels were not sensitive to changes in oxygenpercentages ranging from 21.5 to 1.4%. Fluorescence emission of thexerogel changed significantly (by a factor of 1.8) when in the completeabsence of oxygen.

Quenching processes decrease the emission intensity of fluorescentspecies. In collisional (dynamic) quenching, the excited fluorophoretransfers its excess energy via non-radiative pathways to a quencherbefore it has the opportunity to emit a photon via fluorescence. Oxygenis an efficient collisional quencher, diffusing to the fluorophoreduring the lifetime of the excited state (Lakowicz, Principles ofFluorescence Spectroscopy, 2nd Ed., Kluwer Academic/Plenum Publishers(1999), which is hereby incorporated by reference in its entirety).

The Stern-Volmer equation (shown below) can be used to generatecalibration curves (Lakowicz, Principles of Fluorescence Spectroscopy,2nd Ed., Kluwer Academic/Plenum Publishers (1999), which is herebyincorporated by reference in its entirety).F _(o) /F=1+K _(SV) [Q]  Eq. (2)where F_(o) is the fluorescence of a species in the absence of quencher;F is the fluorescence of that species in the presence of quencher; [Q]is the concentration of the quencher; and K_(SV) is the Stern-Volmerquenching constant. A Stern-Volmer plot, F_(o)/F v. [Q], is linear ifthe fluorophore is present in a single microenvironment. A nonlinearStern-Volmer plot can indicate that the fluorophore has partitioned intodifferent environments with different quencher accessibilities(Lakowicz, Principles of Fluorescence Spectroscopy, 2nd Ed., KluwerAcademic/Plenum Publishers (1999), which is hereby incorporated byreference in its entirety).

Calibration curves for the more promising sensor materials were producedusing the Stern-Volmer Eq. (1). FIG. 13 shows the Stern-Volmer plots forthe Ru(dpp)₃ ²⁺-doped materials: an aerogel, a xerogel, and a post-dopedxerogel. For the aerogel and xerogel, it is clear that the plots ofF_(o)/F vs. % O₂ are not linear, even at low % O₂. Hence, theoxygen-sensitive Ru(dpp)₃ ²⁺ is present in at least two differentmicroenvironments with differing oxygen accessibility. The Stern-Volmerplot of the post-doped xerogel has increased linearity when compared tothe aerogel and the xerogel prepared by adding to the precursor mixtureprior to gelation.

A Stern-Volmer plot of the PtOEP-doped aerogel data (FIG. 14) indicatesthat the response of the sensor is approximately linear for the 0.0% to5.5% O₂ range. At higher % O₂, it becomes obvious that the Stern-Volmerplot is not linear, indicating that PtOEP is present in at least twomicroenvironments within the aerogel.

As a result of multi-site binding, nonlinear Stern-Volmer plots arecommon for sol-gel-based sensors (Leventis et al., Chem. Mater. 16:1493(2004); Watkins et al., Appl. Spectrosc. 52:750 (1998); Baker et al., J.Sol-Gel Sci. Technol. 17:71 (2000); Tang et al., Anal. Chem. 75:2407(2003), each of which is hereby incorporated by reference in itsentirety). A two-site quenching model developed by Demas et al. (Anal.Chem. 67: 1377 (1995), which is hereby incorporated by reference in itsentirety) was employed in modeling this data. If a single sensor speciesexists in two sites, each with a different quenching constant, thefollowing relationship applies:

$\begin{matrix}{\frac{F_{0}}{F} = \frac{1}{\frac{f_{01}}{1 + {K_{{SV}\; 1}\lbrack Q\rbrack}} + \frac{f_{02}}{1 + {K_{{SV}\; 2}\lbrack Q\rbrack}}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$where F₀₁ and f₀₂ are the fractional contributions of the probe in eachsite to the unquenched steady-state emission (f₀₁+f₀₂=1); K_(SV1) andK_(SV2) are the quenching constants for the two different sites (Demaset al., Anal. Chem. 67: 1377 (1995), which is hereby incorporated byreference in its entirety). The program Kaleidagraph was used to fit thedata presented in FIGS. 12 and 13 (Stern-Volmer plots for thefluorophore-doped materials) to the two-site quenching model. In allcases, very good fits were obtained (R²=0.992). The parameters obtainedfrom the curve-fits are listed in Table 1 below.

TABLE 1 Effect of matrix on the oxygen quenching of probe-dopedmaterials Post-doped Ru(dpp)₃ ²⁺-doped Ru(dpp)₃ ²⁺-doped Ru(dpp)₃ ²⁺PtOEP-doped Aerogel Xerogel Xerogel Aerogel f₀₁ 0.30 ± 0.01 0.050 ±0.007 0.274 ± 0.008   0.18 ± 0.01 K_(SV1) (% O₂)⁻¹ 0.033 ± 0.003 *−0.003 ± 0.004     0.015 ± 0.002 * −0.005 (± 0.002) f₀₂ 0.70 0.95 0.7260.72 K_(SV2) (% O₂)⁻¹ 14 ± 5  0.66 ± 0.02 10. ± 2     0.53 ± 0.03 R²0.992 1.000 0.996 0.999 * = the value obtained from these fits indicatesthat the probe in one microenvironment is not significantly quenched byoxygen (i.e. K_(SV1) ≈ 0); Note that values for f₀₂ were calculated bysubtracting f₀₁ from 1.

As demonstrated above, aerogel-platform optical oxygen sensors can beprepared rapidly and inexpensively using the process of the presentinvention. The spectral data indicate small changes in themicroenvironment experienced by the ruthenium complexes relative tosolution, but there does not appear to have been significant degradationof the probes. It is important to note that the probes are exposed tohigh temperatures during the aerogel formation process, because thesupercritical temperature of the methanol-water mixture must be reachedin order for the solvents to be extracted from the sol-gel matrix. Thus,immobilization of thermally unstable probes is unlikely to be achieved.

For this initial demonstration of aerogel-platform gas sensors, specieswere selected that were expected to be relatively stable at thetemperatures employed. If lower temperatures can be used or if theprocess can be optimized to reduce the amount of time spent above thesupercritical temperature of methanol, it will increase the number ofviable probes. Moreover, the high temperatures required for aerogelformation via the process of the present invention might be advantageousin certain cases. For instance, Baker et al. demonstrated that Ru(dpp)₃²⁺-doped sol-gel thin films cured at relatively higher temperatures(>150° C.) were more sensitive to oxygen than films prepared at lowertemperatures (J. Sol-Gel Sci. Technol. 17:71 (2000), which is herebyincorporated by reference in its entirety). This effect was due, inpart, to the dissociation of water from residual silanol groups in thesol-gel at higher temperatures (Baker et al., J. Sol-Gel Sci. Technol.17:71 (2000), which is hereby incorporated by reference in itsentirety).

The porous silica aerogels prepared by the process of the presentinvention made excellent platforms for gas sensors. The three testedoxygen-sensitive species each maintained sensitivity to oxygen whenimmobilized in TMOS-based aerogel monoliths. Oxygen diffused rapidlythrough the aerogel matrix, and interacted with entrapped indicators.Response times were comparable to those observed by Leventis et al. fora covalently attached probe (Leventis et al., Chem. Mater. 11:2837(1999), which is hereby incorporated by reference in its entirety) andfor an aerogel post-doped with a ruthenium complex (Leventis et al.,Chem. Mater. 16:1493 (2004), which is hereby incorporated by referencein its entirety). Because the gas mixing system described above requiredmanual adjustment of the N₂ (g) and air proportions, response timesreported herein are likely longer than the actual response of thesensor.

It is likely that some probes will be immobilized in a sufficientlyrigid manner so as to render them unsuitable as indicators for gas-phaseanalytes in sensor applications. Indeed, the relatively small change insignal (as shown in FIG. 10) limits the practical applicability of theRu(bpy)₃ ²⁺-doped aerogels as oxygen sensors. Preliminary fluorescencelifetime measurements indicate that there are two lifetimes for Ru(bpy)₃²⁺ in the aerogel: one is unusually short (<5 ns) and does not changesignificantly as oxygen is removed from the aerogel; the other lifetimeis approximately 770 ns in air, and 1580 ns in 100% nitrogen. The probein the first microenvironment (with short lifetime) is, presumably,transferring energy to the silica matrix rapidly enough that oxygenquenching is not observed. Moreover, a substantial proportion of theprobes within the aerogel matrix exhibit the shorter lifetime. We arecontinuing fundamental studies of the probe in these silica aerogels andthe corresponding xerogels.

For the Ru(dpp)₃ ²⁺-doped aerogels and xerogels, the dramatic increasein fluorescence intensity coincident with the removal of ambient O₂ (g)is consistent with the literature (Leventis et al., Chem. Mater. 16:1493(2004); Watkins et al., Appl. Spectrosc. 52:750 (1998)); Baker et al.,J. Sol-Gel Sci. Technol. 17:71 (2000); Cho et al., Anal. Chem. 73:3289(2001); Tang et al., Anal. Chem. 75:2407 (2003); Carraway et al., Anal.Chem. 63:337 (1991), each of which is hereby incorporated by referencein its entirety). This oxygen sensitivity makes Ru(dpp)₃ ²⁺ anattractive probe for use as a gas sensor in an aerogel platform.However, the Stern-Volmer plots (FIG. 13) for the Ru(dpp)₃ ²⁺-dopedaerogels and corresponding xerogels indicate quite clearly that theRu(dpp)₃ ²⁺ probe exists in more than one microenvironment, each withdiffering accessibility to oxygen quenching. This behavior is consistentwith previous work by Bright and coworkers (Watkins et al., Appl.Spectrosc. 52:750 (1998); Baker et al., J. Sol-Gel Sci. Technol. 17:71(2000); Tang et al., Anal. Chem. 75:2407 (2003), each of which is herebyincorporated by reference in its entirety). When the data presented inFIG. 13 are fit to a two-site model (Table 1), it becomes clear that themicroenvironments experienced by Ru(dpp)₃ ²⁺ in the aerogel, xerogel,and post-doped xerogel differ.

When Ru(dpp)₃ ²⁺ is entrapped in an aerogel or post-doped xerogel, itexists in at least two distinct microenvironments: ˜30% of the signal inthe absence of oxygen is due to the probe in an environment of lowoxygen-sensitivity (K_(SV1)<0.05% O₂ ⁻¹), whereas about 70% of the probeis in a highly oxygen sensitive environment (K_(SV2)>10% O₂ ⁻¹). Incontrast, the xerogel prepared from the same precursor mixture as theaerogel exhibits markedly different quenching behavior, with the vastmajority of the signal responsive to changes in oxygen concentration,but with a lower K_(SV) than for the other materials. The presence ofeven small amounts of oxygen reduces the fluorescence intensity of eachof the materials considerably, so these monoliths are well suited toapplication as switches. Preliminary fluorescence lifetime measurementsindicate that there are three different fluorescence lifetimes forRu(dpp)₃ ²⁺ in the gels, indicating three microenvironments.

The PtOEP-doped aerogels are suitable for quantitative sensorapplications. The response time is comparable to thin-film sol-gel workby others (Del Monte et al., Langmuir 16:7377 (2000), which is herebyincorporated by reference in its entirety). A factor of 3.7 increase insignal is observed when the sensor cycles from air to 100% nitrogen.This sensor is less sensitive than some reported in the literature. WhenAmao et al. immobilized PtOEP in a thin film ofpoly(styrene-co-pentafluorostyrene) (Amao et al., Analyst 125:1911(2000), which is hereby incorporated by reference in its entirety byreference), they obtained an F₀/F ratio of 18.0 for the fluorescence ofPtOEP in 100% argon (0% oxygen) to 100% oxygen. From the fit of thetwo-site model to the obtained quenching data for the PtOEP-dopedaerogel (Table 1), it is apparent that a significant fraction (ca. 20%)of the PtOEP is entrapped in a microenvironment in which it isinaccessible to oxygen quenching.

From the foregoing, it should be appreciated that the Ru(dpp)₃ ²⁺-dopedaerogels and xerogels show promise for use as switches, and thePtOEP-doped aerogels have potential application as quantitative sensors.

While these aerogels are very promising materials for sensorapplications, they have been shown to collapse in water. This behaviorlimits the potential applications of these aerogel-platform O₂ (g)sensors, particularly in the field of biosensors, as most biologicalsystems are aqueous. The RSCE process of the present invention has notbeen fully optimized, and it is possible that optimization of processingparameters will allow more control over the resulting aerogelproperties, including the ability to prepare more rugged materials. Itwill be important to assess the long-term effects of humidity andtemperature on sensor stability.

To date, no attempt was made to control the humidity or temperature ofthe aerogels; they were stored in capped polystyrene cuvettes at roomtemperature, in a building with only moderate climate control. The sameRu(dpp)₃ ²⁺-doped aerogels were used over the course of ten months,through four seasons, and no obvious degradation of signal intensity oroptical clarity was observed over that time. The doped aerogels areclearly stable; although more controlled studies of temperature andhumidity effects are warranted.

Based on the above-described results, it is expected that other probescan be doped into the aerogels to form sensors specialized for othergas-phase molecules, such as carbon monoxide, carbon dioxide, andhydrocarbons.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions, and the like canbe made without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the claims which follow.

1. A method for rapidly producing aerogels comprising: heating andapplying external restraining force to a sealed, gas impermeable vesselthat contains therein a gel precursor solution or a pre-formed gel thatat least partially fills the internal volume of the sealed vessel, saidheating and applying external restraining force being carried outwithout substantial venting of the vessel and thereby confining physicalexpansion of the gel or gel precursor; and controllably releasing theexternal restraining force applied to the vessel, thereby allowing forventing and release of internal pressure to form the aerogel.
 2. Themethod according to claim 1 wherein said heating and applyingrestraining force is carried out simultaneously.
 3. The method accordingto claim 1 wherein the applied external restraining force is at leastabout 70 kN.
 4. The method according to claim 1 wherein said heatingcomprises heating the sealed vessel at a rate of about 1° C. to about 2°C. per minute.
 5. The method according to claim 1 further comprising:maintaining the temperature and restraining force applied to the sealedvessel for a dwell time prior to said controllably releasing.
 6. Themethod according to claim 5 wherein said maintaining comprises a dwelltime of about 1 to about 60 minutes.
 7. The method according to claim 6wherein said maintaining comprises a dwell temperature of about 240 toabout 300° C.
 8. The method according to claim 1 wherein saidcontrollably releasing comprises removing a portion of the restrainingforce applied to the sealed vessel.
 9. The method according to claim 8wherein said removing is carried out whereby the remaining force appliedto the sealed vessel via the hot plates is at least about 4 kN.
 10. Themethod according to claim 1 wherein the gel or gel precursor comprisesone or more chemical sensing agents.
 11. The method according to claim10 wherein the chemical sensing agent is a fluorescent dye or afluorescent coordination complex.